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Conifers exhibit a number of constitutive and induced mechanisms to defend against attack by pests and pathogens such as mountain pine beetle (Dendroctonus ponderosae Hopkins) and their fungal associates. Ecological studies have demonstrated that stressed trees are more susceptible to attack by mountain pine beetle than their healthy counterparts. In this study, we tested the hypothesis that water deficit affects constitutive and induced responses of mature lodgepole pine × jack pine hybrids (Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Wats. × Pinus banksiana Lamb.) to inoculation with the mountain pine beetle fungal associate Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield. The degree of stress induced by the imposed water-deficit treatment was sufficient to reduce photosynthesis. Grosmannia clavigera-induced lesions exhibited significantly reduced dimensions in water-deficit trees relative to well-watered trees at 5 weeks after inoculation. Treatment-associated cellular-level changes in secondary phloem were also observed. Quantitative RT-PCR was used to analyze transcript abundance profiles of 18 genes belonging to four families classically associated with biotic and abiotic stress responses: aquaporins (AQPs), dehydration-responsive element binding (DREB), terpene synthases (TPSs) and chitinases (CHIs). Transcript abundance profiles of a TIP2 AQP and a TINY-like DREB decreased significantly in fungus-inoculated trees, but not in response to water deficit. One TPS, Pcb(+)-3-carene synthase, and the Class II CHIs PcbCHI2.1 and PcbCHI2.2 showed increased expression under water-deficit conditions in the absence of fungal inoculation, while another TPS, Pcb(E)-β-farnesene synthase-like, and two CHIs, PcbCHI1.1 and PcbCHI4.1, showed attenuated expression under water-deficit conditions in the presence of fungal inoculation. The effects were observed both locally and systemically. These results demonstrate that both constitutive and induced carbon- and nitrogen-based defenses are affected by water deficit, suggesting potential consequences for mountain pine beetle dynamics, particularly in novel environments.

North temperate tree species such as white spruce (Picea glauca [Moench] Voss) have evolved strategies to protect themselves against abiotic and biotic stresses that trees encounter during the inclement winter months. Chitinases not only play well-documented roles in plant defense but also function during physiological and developmental preparations for overwintering, including growth cessation, cold and desiccation acclimation, and dormancy acquisition. Phylogenetic analysis of 31 white spruce and 52 Norway spruce chitinases identified genes falling into each of the five clusters, which sometimes—but not always—separated the different biochemical classes of chitinases. Digital expression profiling of white spruce and Norway spruce chitinases across multiple conditions revealed a range of spatiotemporal expression patterns. Transcript abundance profiling in buds, needles, stems, and roots by quantitative RT-PCR suggested roles for eight white spruce chitinases during the growth-to-dormancy transition. In silico analyses of these eight sequences suggested that two cluster 2/class I chitinases function as chitinolytic enzymes in the tree’s constitutive defense arsenal during the winter months. A cluster 2/class I, cluster 2/class II, and cluster 1/class IV chitinase each exhibit hallmarks of antifreeze proteins. Additionally, two cluster 2/class I chitinases and a cluster 1/class IV chitinase may serve as vegetative storage proteins. One cluster 3/class II chitinase exhibited attributes suggesting that it is a chitinase-like gene functioning in cell wall synthesis. Taken together, our results imply that dormancy-associated chitinases act in concert to (1) confer protection against freezing injury, pests, and pathogens, (2) store nitrogen, and (3) promote cell maturation that precedes growth cessation.

In the autumn, stems of woody perennials such as forest trees undergo a transition from active growth to dormancy. We used microarray transcriptomic profiling in combination with a proteomics analysis to elucidate processes that occur during this growth-to-dormancy transition in a conifer, white spruce (Picea glauca[Moench] Voss). Several differentially expressed genes were likely associated with the developmental transition that occurs during growth cessation in the cambial zone and the concomitant completion of cell maturation in vascular tissues. Genes encoding for cell wall and membrane biosynthetic enzymes showed transcript abundance patterns consistent with completion of cell maturation, and also of cell wall and membrane modifications potentially enabling cells to withstand the harsh conditions of winter. Several differentially expressed genes were identified that encoded putative regulators of cambial activity, cell development and of the photoperiodic pathway. Reconfiguration of carbon allocation figured centrally in the tree's overwintering preparations. For example, genes associated with carbon-based defences such as terpenoids were down-regulated, while many genes associated with protein-based defences and other stress mitigation mechanisms were up-regulated. Several of these correspond to proteins that were accumulated during the growth-to-dormancy transition, emphasizing the importance of stress protection in the tree's adaptive response to overwintering.

Background Flax (Linum usitatissimum L.) is an important crop for the production of bioproducts derived from its seed and stem fiber. Transposable elements (TEs) are widespread in plant genomes and are a key component of their evolution. The availability of a genome assembly of flax (Linum usitatissimum) affords new opportunities to explore the diversity of TEs and their relationship to genes and gene expression. Results Four de novo repeat identification algorithms (PILER, RepeatScout, LTR_finder and LTR_STRUC) were applied to the flax genome assembly. The resulting library of flax repeats was combined with the RepBase Viridiplantae division and used with RepeatMasker to identify TEs coverage in the genome. LTR retrotransposons were the most abundant TEs (17.2% genome coverage), followed by Long Interspersed Nuclear Element (LINE) retrotransposons (2.10%) and Mutator DNA transposons (1.99%). Comparison of putative flax TEs to flax transcript databases indicated that TEs are not highly expressed in flax. However, the presence of recent insertions, defined by 100% intra-element LTR similarity, provided evidence for recent TE activity. Spatial analysis showed TE-rich regions, gene-rich regions as well as regions with similar genes and TE density. Monte Carlo simulations for the 71 largest scaffolds (≥ 1 Mb each) did not show any regional differences in the frequency of TE overlap with gene coding sequences. However, differences between TE superfamilies were found in their proximity to genes. Genes within TE-rich regions also appeared to have lower transcript expression, based on EST abundance. When LTR elements were compared, Copia showed more diversity, recent insertions and conserved domains than the Gypsy, demonstrating their importance in genome evolution. Conclusions The calculated 23.06% TE coverage of the flax WGS assembly is at the low end of the range of TE coverages reported in other eudicots, although this estimate does not include TEs likely found in unassembled repetitive regions of the genome. Since enrichment for TEs in genomic regions was associated with reduced expression of neighbouring genes, and many members of the Copia LTR superfamily are inserted close to coding regions, we suggest Copia elements have a greater influence on recent flax genome evolution while Gypsy elements have become residual and highly mutated.